The concept of sources of law has only for a short time in history been confined to the formally adopted legal norms of statutory, conventional or customary nature. Such a positivistic and narrow notion can be confronted to an older layer of the concept, which has gained again some prominence in the twentieth century, whereby the sources encompass also all the elements which concur to the interpretation and development of the law. In this larger area, general principles of law play an important, albeit often invisible, role as catalysts of a law-in-movement. In particular, they offer a legal basis from which new legal doctrines or norms can be derived from case to case, in order to fit new social and legal needs. The evolution thereby remains at least partly anchored in the legal sphere, through the link with the legally connoted general principles, and at the same time strikes out to the changing needs in the social and political arena. Thus, general principles of law are particularly instructive elements of this type of dynamic equilibrium to be achieved. Their different functions, and especially their role as law-creating arguments, are put into limelight in this short contribution. These functions are illustrated more concretely through the example of the powerful principle of good faith.